What is the Sharia ruling on performing a fetal reduction operation after the division of one of the implanted embryos—given that the mother does not suffer from any health problems—and is refraining from doing so considered a sin?

Aborting a fetus before the ensoulment (i.e., before completing 120 days) is a matter of disagreement among scholars. Some have prohibited it, while others have permitted it for an excuse.

Although this opinion is not to be universally favored, it is permissible to adopt it when there is an excuse and a need, such as fear of an incomplete pregnancy, delayed fetal growth, entering into problems of premature birth, or the hardship that afflicts the mother from giving birth to triplets and the anticipated harm to the children themselves. All of these are considered excuses that permit aborting one of the fetuses before ensoulment.

If abortion is permissible in this case, then there is no objection to the will of Allah Almighty in that; rather, it is a form of seeking treatment and taking means.

As for refusing the reduction procedure, there is no sin upon the woman in that.